The present invention relates generally to integrated circuit manufacturing, and more particularly to, a method for forming tungsten contact plugs in a void-free manner.
In the integrated circuit (IC) industry, contacts between polysilicon layers and/or metal layers are sometimes formed using tungsten plugs. As advances are made in lithographic processing and etch technology, the radial opening of an integrated circuit contact continues to shrink while the vertical depth of integrated circuit contacts have been increasing. This increase in contact aspect ratio (depth/radius) creates reliability and manufacturing problems when using tungsten deposition and chemical mechanical polishing (CMP) technology to form tungsten plugs.
The problems arise when tungsten (W) continues in the industry to be used to plug large aspect ratio contacts is illustrated in prior art FIGS. 1-3. As is known in the art, tungsten deposition suffers from highly nonconformal deposition characteristics, also referred to as inconsistent step-coverage. The adverse effects of this non-conformal nature of tungsten deposition is illustrated in FIGS. 1-3.
In FIG. 1, a contact structure 10 is formed by initially providing a base layer 12. A first conductive layer 14 is formed on top of the base layer 12. Layer 14 is lithographically patterned and etched. A dielectric layer 16 is formed overlying the patterned layer 14. The dielectric layer 16 is then patterned then etched to form a contact opening 18 which has a substantially uniform radius X at all elevations through the contact 18 of FIG. 1. FIG. 1 illustrates the beginning of the tungsten deposition process which forms an initial tungsten layer 20a. 
In FIG. 1, tungsten is deposited in a highly nonconformal manner. In other words, top surfaces of the dielectric layer 16 will accumulate tungsten material at a much faster rate than the bottom comers of the contact opening 18. Therefore, the shape of the initial stages of tungsten deposition is accurately illustrated in FIG. 1 whereby top portions of the dielectric layer 16 have accumulated a greater thickness of tungsten than lower portions of the contact opening 18.
In FIG. 2, tungsten deposition continues to transform the thinner tungsten layer 20a in FIG. 1 to a thicker tungsten layer 20b in FIG. 2. The tungsten, which continues to deposit in FIG. 2, is also nonconformal and deposits more along the top exposed surfaces of layer 20a and less along the sidewalls and bottom portion of the contact 18. Due to this nonconformal deposition, many contacts formed using a tungsten deposition process will form keyholes or voids 22 as illustrated in FIG. 2. These voids form from the nonconformal deposition nature of tungsten xe2x80x9cpinching offxe2x80x9d the top opening in the contact hole. The voids 22 resulting from this nonconformal step coverage of tungsten create depressed yields, nonfunctional IC die from electrical open circuits, and electromigration failures over time. Therefore, the presence of the voids 22 in high aspect ratio contacts are disadvantageous and a significant problem for integrated circuit (IC) processing.
FIG. 3 illustrates that a chemical mechanical polishing (CMP) operation is used to form the tungsten plug 20c illustrated in FIG. 3. Note that the void 22 is still present within the contact structure after polishing and therefore is still problematic in the final integrated circuit (IC) device.
Therefore, a need exists for a IC contact formation process which continues to utilize nonconformal tungsten deposition processes while resulting in reduced or totally eliminated void formation.